In the past, various constructions of a somewhat low profile headrail have been developed by Levolor Corporation and other venetian blind manufacturers. A first example of a low profile headrail particularly designed for installation between double glazed windows is a standard venetian blind with a headrail having a dimensional profile of about 2 cm high by 3 cm deep where a tilt rod in the headrail is driven by a flexible shaft connected to a blind-exterior knob assembly. The blind is sealed within the window glazing and controlled by the external knob. Since this construction incorporates a typical drum and cradle assembly such as shown in U.S. Pat. No. 4,487,243, the headrail must be sufficiently large in height so as to accept the operating hardware. A sleeker profile, evidenced by the TCR Andersen Window Blind by Levolor, eliminates the use of drums and cradles, and replaces them with a system of stainless steel wires, terminals, and pulleys. In this construction, the upper legs of the ladder tape are secured to a steel cable with brass barbs, which pass through terminal ends. Tilt cords are attached by barbs and eyelets to either end of the steel cable, which then passes around a roller-bearing pulley. Slat tilt is accomplished by pulling on the tilt cords. Although, in this manner, the dimensional profile of the headrail can be significantly reduced, the construction does not permit the use of a conventional tilter and tilt wand. Further, assembly of the product is particularly difficult with regard to accurately measuring and attaching the terminal ends. A partial solution to this problem is seen in U.S. Pat. No. 4,945,971 where a ladder cord has pre-formed apertures in the ladder legs and pins utilized to fix the leg to a bent tongue on the drum or in a bottom rail aperture.
There thus has been the need for a low profile headrail which can be manufactured and assembled with ease and suitable for incorporation into a double-glazed window assembly or used in a standard window. This would result in enhanced functional operation by quickly, dependably and smoothly tilting the blind slats while providing for improved operation of the draw cords for lowering and raising and locking a stack of slats making up the blind.
Most venetian blinds of the prior art have headrails having a height of from about 2 cm to 2.5 cm, with a depth of from about 3 cm to 4 cm. Such large dimensioned headrails cannot be installed on restricted mounting surfaces nor would they fit within the narrow horizontal air space typical of double glazed windows.
Chain drives, exemplified by U.S. Pat. Nos. 4,214,622 and 4,621,273 have been employed in headrails to effect the tilting of the blind slats. The chains are driven by a sprocket. Such designs and other standard tilt mechanisms involve a large amount of sub-assembly and a large number of parts to construct the product. For example, in the cable-cord tilter system described above, a total of 43 parts are employed in a two-ladder blind, 55 parts in a three-ladder blind and 67 parts in a four-ladder blind. The number of ladders are dictated by the overall width of the blind and slats and the number chosen prevents the sagging of the normally employed thin metal slats forming the blind.
A problem in blinds having a low profile is that the draw cords which pass through slat apertures from an anchored position in a blind bottom rail must pass laterally through the headrail to an end locking mechanism and cord outlet and thence alongside the blind slat ends for manipulation by a user. Generally headrail mounting screws or other fasteners pass freely through the headrail and are threaded or connected into a wood or other window upper horizontal frame portion. Often the installation screws pinch and cut or fray the lift or draw cords as the lift cords move in the headrail. This problem is amplified in small, low profile headrails due to the close clearances therein.
Further, as the height of a headrail becomes smaller it is more difficult to accommodate the conventional cord lock employed in venetian blinds where the cords are passed over a horizontal roller or pin and then passed down vertically through the cord lock. Such cord locks as exemplified in U.S. Pat. No. 3,799,236 rely on gravitational forces to move a roller cam into position against the draw cords with the cam's knurled surface locking or jamming the cord securely against the roller or pin. This construction necessitates a relatively high headrail. Thus there has been a need to have a cord lock which can effectively operate in a low profile headrail.